One type of transdermal drug delivery device is in the form of a patch applied to the subject's skin and containing drug penetrating the skin by osmosis and/or by a controlled mass transport phenomenon such as iontophoresis. Simple patches, however, provide no control, or limited control, of the rate of drug delivery, which depends on skin conditions, the nature (particularly molecular size) of the drug to be delivered, and the like. Iontophoretic devices are also not entirely satisfactory in their ability to deliver large molecules and to control the rate of delivery thereof. All such devices are limited by the barrier function of the skin.
Another transdermal drug delivery device is described in International Patent Publication WO 93/17754. In one embodiment this device comprises a housing containing a liquid reservoir and a drug delivery body carried by the housing and engageable with the subject's skin. The drug delivery body carries a plurality of hollow needles (of which there are preferably at least fifty) having an outer diameter of the order of 1 mm, which needles are designed to pierce the outer layer of dead cells (the stratum corneum) of the skin, thereby enhancing the penetration of the drug through the skin.
However, certain disadvantages are associated with this method of drug delivery. Firstly, there is a risk of considerable pain and traumatisation of the skin associated with the application of the particular array of needles. Secondly, the drug may leak out around the entry point of each needle as a result of the pressure being applied to assist the delivery of the drug. A film of liquid drug covering the area of application may cause irritation for subjects with sensitive skin; certain drugs may aggravate this irritation. The leakage also results in a lower efficiency of drug delivery. Thirdly, it can be difficult to ensure that the device is correctly applied with the tips of the needles penetrating the strateum corneum. The skin has a natural resilience and elasticity. The device is pressed onto the skin such that the entire area of the needle arrangement depresses the surface of the skin, even when considerable pressure is applied. For this reason, an extra degree of pain is associated with the correct application of the device due to the amount of force needed to properly pierce the stratum corneum with all of the needles.